1. Field of the Invention
The present invention relates to a permselective thin membrane prepared from an organic polyamide or polyhydrazide crosslinked by a polyvalent metal. Such membranes are useful for reverse osmosis or ultrafiltration. More particularly, this invention is useful for desalination of sea water or brackish water, and functions to separate waste materials which are the origin of environmental pollution from waste waters, such as products of the dye or pulp industries, for example.
2. Description of the Prior Art
The term "osmosis" is commonly used to describe the spontaneous flow of pure water into saline water, where pure water and saline water are separated by a suitable semi-permeable membrane.
The term "reverse osmosis" is used to describe water flow from saline water into pure water through a membrane when hydrostatic pressure is applied to the saline water in excess of the equilibrium osmosis pressure. When the saline solution is in contact with a permselective membrane the saline water actually becomes more concentrated. Generally, this reverse osmosis process is useful to desalinate sea water and may be applied to waste water treatment and to concentration of food. In the separation of water from such an aqueous solution, the reverse osmosis process has several advantages including operation with low energy expenditures, and operation at room temperature. This is because the water undergoes no phase change, such as from liquid to vapor, as may occur in a distillation process, for example.
Accordingly, the reverse osmosis process is particularly useful for separating materials which are sensitive to deterioration by heat. This process also has merit in that miniaturization of equipment is possible. So it may be said that the reverse osmosis process is very promising compared to other processes, and extensive research has been performed on it in the United States.
It is known that a cellulose acetate membrane, developed by Loeb and Coworkers and described in U.S. Pat. Nos. 3,133,132 and 3,133,137 is useful for carrying out the reverse osmosis process. However, cellulose acetate membranes have serious deficiencies in that they have a limited operating life and are easily hydrolyzed (deacetylated) during operation. This prohibits their widespread application.
In recent years, alternate permselective membrane materials have been actively sought and suggested. For example, membranes made from polyacrylonitrile, polyhydroxyethylacrylate, polyvinylalcohol and polyvinylidene carbonate have been suggested. However, these membranes are inferior to cellulose acetate membranes in permselectivity and in water permeability as well.
More recently, the applications of new polymeric materials developed for thermalstable polymers such as aromatic polyamides, polyamide-hydrazides, polysulphones, polyphenylene oxides, for permselective membranes, have been actively tried. For example, aromatic polyamide and polyamide-hydrazide membranes are described in U.S. Pat. Nos. 3,567,632 and in Am. Chem. Soc. Polymer Preprints 16, 2,365,385 (1972). During operation these membranes exhibit improved properties, both mechanical and chemical, in comparison with cellulose acetate. However, they have "low water flux" because of their low permeability to water.
Polysulphones and polyphenylethers themselves are not good materials for reverse osmosis, but introduction of hydrophilic groups into the polymer main chain makes them useful as permselective membranes, according to the description in Am. Chem. Soc. Polymer Preprints 16, No. 2,276 (1972).
In spite of their improved mechanical and chemical properties, however, their permselectivity and permeability are still inadequate in comparison with cellulose acetate. Accordingly, efforts have been made without success to obtain a membrane having high water-flux and low salt passage, and having improved mechanical and chemical properties.
New series of polyamides and polyhydrazides which contain pendant ionic groups in their polymer chains were developed in order to attain unexpectedly higher water flux membranes (U.S. patent application Ser. No. 472,446, now U.S. Pat. No. 3,993,625).
An object of the present invention is to improve both the water flux and the durability of a membrane such as just described above.
Water flux depends upon hydrostatic pressure, solute concentration (osmotic pressure), temperature and also compaction of membrane due to applied pressure. The flux decline, expressed in terms of operation time, is expressed as follows: EQU log x = log A + m log T
where x and A are water flux after one day and after T days, respectively, and m is a parameter of flux change. The m-value is usually a little below zero, which means a gradual decrease of flux with time. In order to realize the most economical operation, the m-value should be zero or nearest to zero.
One way to improve the m-value is to crosslink the membrane polymer, which makes the membrane durable against creep. One of such examples is crosslinking of cellulose acetate by incorporating cellulose methacrylate (Polymer Preprints 12 No. 2, 284 (1971)). That procedure, however, provides improvement of m-value at the expense of water flux.
The present invention contributes a significant improvement of m-value together with water flux increase only through crosslinking polyamides and polyhydrazide with polyvalent ions.